The present invention relates to a process for the transesterification of keto esters with alcohols using polyaniline salts as catalyst.
Transesterification is a reaction between an ester and other compound, characterized by an exchange of alkoxy groups or of acyl groups and resulting in the formation of a different ester. Three types of transesterification are known (Kirk-Othmer Encyclopedia of Chemical Technology (Ed., Jacqueline I. Kroschwitz), 4th Edition, Vol.9, Page 774 and references therein)
1. Exchange of alcohol groups, commonly known as alcoholysis. In this process the compound with which an ester reacts is an alcohol.
Rxe2x80x94COOxe2x80x94R1+R2xe2x80x94OHxe2x86x92Rxe2x80x94COOxe2x80x94R2+R1xe2x80x94OH
2. Exchange of acid groups, acidolysis. In this process the compound with which an ester reacts is an acid
Rxe2x80x94COOxe2x80x94R1+R2xe2x80x94COOHxe2x86x92Rxe2x80x94COOH+R2xe2x80x94COOxe2x80x94R1
3. esterxe2x80x94ester interchange. In this process an exchange takes place between two esters
Rxe2x80x94COOxe2x80x94R1+R2xe2x80x94COOxe2x80x94R3xe2x86x92Rxe2x80x94COOxe2x80x94R3+R2xe2x80x94COOxe2x80x94R1
A normal method of transesterification is characterized by equilibrium between reactants and products. Generally, one of the reactants should be added in excess to move the reaction towards forward direction and obtain good yields.
Transesterification is more advantageous than the ester synthesis from carboxylic acid and alcohol, due to poor solubility of some acids in organic solvents. Some esters, especially methyl and ethyl esters, are readily or commercially available and thus serve conveniently as starting materials in transesterification.
Transesterification via alcoholysis plays a significant role in industry as well as in laboratory and in analytical chemistry. The reaction can be used to reduce the boiling point of esters by exchanging a long chain alcohol group with a short one e.g. methanol, in the analysis of fats, oils and waxes. Transesterification is applicable in the paint industry for curing alkyl resin. It plays an important role in polymerization and in cosynthesis of ethylene glycol and dimethyl carbonate from ethylene carbonate and methanol.
xcex2-Ketoesters represents an important class of organic building blocks and is used for efficient synthesis of a number of complex natural products. xcex2-Ketoesters are multicoupling reagents with electrophilic and nucleophilic sites proven to be valuable tools in a wide variety of molecular systems. These xcex2-ketoesters are important by virtue of their facile bond formation at all four carbon atoms that feature in their ease of transformation to chiral building blocks and use in chain extension reactions. They are one of the basic building blocks in the total synthesis of sex pheromones like serricornine and other natural products like thiolactomycin, trichodiene, polyoximic acid, chokol, protaglandin PGF2xcex1, ar-pseudotsugonoxide, syncarpic acid, diplodialide and podophyllotoxin.
Most of the methods of transesterification of ketoesters are not general and are equilibrium driven reactions where usage of excess of one of the reactants is mandatory to obtain good yields. Process for transesterification of keto esters has been carried out by using various catalysts such as Lewis acids (Holmquist et al. J Org Chem., 1989; 54:3258), Dimethylamino pyridine (Taber et al. J. Org. Chem., 1985; 50: 3618), Iron (III) perchlorate (Kumar et al. Ind. J. Chem., 1993; 32B: 292), Zeolites (Balaji et al. Tetrahedron 1998; 54: 13237), Distannoxane (Otera et al. J. Org. Chem. 1991; 56 (18):5307), Natural kaolinitic clay (Ponde et al. J. Org. Chem. 1998; 63: 1058), Amberlyst-15 (Chavan et al. Synth. Commun. 2001; 31(2): 289), Moxe2x80x94ZrO2 solid acid (Reddy et al. Synth. Commun. 1999; 29 (7): 1235), Sodium perborate (Bandagar et al; Chemistry Letters, 2001; 894), Solid acid catalyst (Chavan et al. U.S. Pat. No. 6,376,701, 2002).
Esterification is one of the most fundamental and important reactions in organic synthesis. Conventionally, the processes of making esters can be classified into the following three main categories:
(a) Liquid-phase esterification reaction utilizing a liquid catalyst: This type of processes utilize liquid phase acid, such as sulfuric acid, phosphoric acid, or sulfonic acid, as catalysts.
(b) Liquid phase esterification reaction utilizing a solid catalyst: This type of processes typically utilizes inorganic salts, cationic ion exchange resin and solid acid catalyst etc.
(c) Gas phase esterification reaction: This type of processes utilize a variety of catalysts such as heteropolyacids, liquid phase acids carried by a solid carrier, and zeolite in a gas phase reaction.
One of the problems associated with the liquid-phase esterification reaction using liquid-catalyst is that the acidic catalysts of sulfuric acid or sulfonic acid can cause corrosion problems to the reactor. These liquid acid catalysts are also discharged along with the reaction products, thus causing severe waste disposal and pollution problems. The drawbacks of using mineral acid as catalyst are: (i)Catalyst can not be reused, (ii) Disposal of acid is not environmentally safe and it is not economical, (iii) Low selectivity is frequently observed, (iv) Corrosion of the reaction vessel and reactors, (v) Not easy to handle and (vi) High inventory of the catalyst.
The solid-catalyst liquid-phase esterification reaction, which typically utilizes a cationic ion exchange resin as catalyst, ameliorates the corrosion and waste disposal problems experienced with the liquid-catalyst liquid-phase processes, and results in simplified separation procedure required between the reaction product and catalysts. However, cationic ion-exchange resins typically exhibit relatively poor heat-resistance, and they often lose substantial activity after being subject to heat. Once the catalytic activity of the cationic ion-exchange resins is reduced, it is difficult to be regenerated.
In the gas phase esterification reaction, the reaction conditions are maintained so that all the reactants and products are in the gas phase. Typically, inorganic materials are utilized as catalysts which typically exhibit excellent heat resistance and can be easily separated from the reaction products. However, the gas phase reaction necessitates a relatively large reaction vessel, resulting in large capital investment cost. Furthermore, if the gas phase esterification reaction is utilized to produce unsaturated carboxylic esters, the high reaction temperature often causes undesired by-products of polymers or oligomers to be produced. In certain instances, the high reaction temperature has caused the alcohol molecules to be dehydrated to become ethers. These side-reactions will tend to cause the reaction catalysts to lose their activity and result in operational difficulties.
The main object of the present invention is to provide a process for the transesterification of keto esters with alcohols using polyaniline salts as catalysts, which obviates the drawbacks as detailed above.
Another object of the invention is to provide an eco-friendly process for the transesterfication of ketoester.
Accordingly, the present invention provides a process for transesterification of ketoester using polyaniline salt as catalyst by reacting said keto ester with an alcohol in presence of a catalyst and separating the esters from the reaction mixture.
Accordingly, the present invention provides a process for transesterification of ketoester using polyaniline salt as catalyst, said process comprising reacting a keto ester with an alcohol in presence of a catalyst at a temperature range of 50 to 120xc2x0 C. for a period in the range of 4 to 24 hours and separating the esters from the reaction mixture.
One embodiment of the invention relates to a process, in which the alcohol used is selected from a group consisting of butanol, hexanol, octanol, decanol, dodecanol, behnyl alcohol, benzyl alcohol, cyclohexanol, 2-ethoxy ethanol, 2-butoxy ethanol, 3-butyne-1-ol, allyl alcohol, and menthol.
Another embodiment of the invention, the catalyst used is a polyaniline salt selected from a group consisting of polyaniline-sulfuric acid, polyaniline-hydrochloric acid and polyaniline-nitric acid system.
Still another embodiment, the preferred reaction temperature is in the range of 100 to 110xc2x0 C.
Still another embodiment, the ketoester used is selected from a group consisting of methyl acetoacetate, ethyl acetoacetate and phenyl acetoacetate.
Yet another embodiment, the reaction is carried out for a period of 20 to 24 hours.
Yet another embodiment, the catalyst amount used is in the range of 10 to 30 weight percent of ketoester
Still yet another embodiment provides a process, wherein the amount of alcohol used is 1.0 to 2.5 equivalent with respect to one equivalent of keto ester.
Another embodiment of the invention, the catalyst used is recycled.
These embodiments will be apparent from the ensuing detailed description of the present invention.
The process of transesterification may be carried out by reacting keto ester with alcohol in presence of catalyst, and then removing the catalyst by conventional methods. The ester can be isolated by methods such as filtration followed by isolation of esters by conventional column chromatography or vacuum distillation.
The novelty of the invention lies in the use of polyaniline-salts as catalysts in the liquid phase esterification of keto esters with alcohols for the first time. Also, the use of polyaniline salts as catalysts provides the following advantages (i) separation of catalyst from a reaction mixture is easy, (ii) repeated use of catalyst is possible and (iii) there is no problem for the disposal of used catalyst as they are environmentally safe.